The present invention relates to friction reducers and methods of using such substances. More particularly, the present invention relates to friction reducers capable of reducing the friction of fluids comprising carbon dioxide.
Viscous gelled fluids are commonly used in the hydraulic fracturing of subterranean zones to increase the production of hydrocarbons from the subterranean zones. That is, a viscous fracturing fluid is pumped through a well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed or enhanced into the subterranean zone. The fractures propagate, e.g., as vertical and/or horizontal cracks radially outward from the well bore.
Such viscous fluids may carry particulate material, such as proppant. When used, the particulate material is suspended in the fluid so that it is deposited in the fractures or along the gravel pack when the viscous fluid is broken and recovered. When used in a fracturing operation, the particulate material functions to prevent the fractures from closing, and thus, aids in forming conductive channels through which produced fluids may flow into the well bore. Without the particulate material, the fractures tend to close and reduce permeability gained by the fracturing operation. Suitable particulate materials must have sufficient compressive strength to resist crushing, but also must be sufficiently non-abrasive and non-angular to preclude cutting and imbedding into the formation.
In carrying out hydraulic fracturing and many other subterranean operations, fluid recovery is critical. Foamed fluids have been developed, inter alia, to provide enhanced fluid recovery through energization by the compressed gas phase. They also reduce the total amount of water used, typically by a factor of about four. Such foamed fluids have included various surfactants, known as foaming and foam stabilizing agents, for facilitating the foaming and stabilization of the foam produced when a gas is mixed with the fracturing fluid. Thus, foamed fluids are media in which a relatively large volume of gas is dispersed in a relatively small volume of liquid, usually with the aid of a surfactant that reduces the surface tension of the fluids. The most commonly used gases for foamed fracture fluids are nitrogen, carbon dioxide, and combinations of the two. Suitable gases generally are non-combustible, readily available, and relatively inexpensive. Carbon dioxide foams may be preferred based on the fact that their use results in little or no loss of hydrostatic pressure since, inter alia, carbon dioxide foam has a density close to water. Moreover, particularly in dry gas reservoirs, the higher density carbon dioxide fluid may be more effective at displacing the load water from the subterranean formation rather than the formation dry gas due to the solubility of carbon dioxide in load waters.
Foamed fluids may be preferred over conventional viscous fluids because they generally provide superior fluid recovery as well as excellent fluid loss control without forming a substantial filter cake. When used in fracturing, foamed fluids generally propagate longer and narrower fractures that remain in the producing zone and are believed to provide superior production enhancement through reduced fracture conductivity damage. Enhanced fluid recovery is provided by the expansion of the gas in the foam when the hydraulic pressure is released after the fracturing operation. This promotes flow of residual fracture fluid liquid back into the well, thus aiding in cleanup of the fracture fluid once the subterranean treatment is complete.
Fracturing techniques using foamed fluids having carbon dioxide as at least part of the gas phase are known in the art. In one common embodiment, a gelled aqueous fluid comprising a surfactant is mixed with liquid carbon dioxide at the surface of the well. The mixture, which is initially an emulsion, generally forms foam as it proceeds down the well bore to the formation to be fractured as the mixture warms to a temperature above the critical temperature of carbon dioxide (about 31° C.). Where the down hole temperature is not above the critical temperature of carbon dioxide, the mixture usually remains an emulsion. Both carbon dioxide emulsions and foams generally are suitable for use as fracturing fluids. As used herein, the term “foamed fluid” will be understood to encompass both true foams and emulsions.
In most cases, carbon dioxide foams and emulsions have deeper well capability than nitrogen foams. Carbon dioxide foamed fluids have greater density than nitrogen gas foamed fluids so that the surface pumping pressure required to reach a corresponding depth is lower with carbon dioxide than with nitrogen. Moreover, when exposed to water, carbon dioxide reacts to form carbonic acid, so that carbon dioxide foams and emulsions generally exhibit a pH in the range of from about 2 to about 6.
When fluids travel through a conduit, such as a pipeline, friction resulting from the movement of the fluid over the inside surface of the conduit may cause a pressure drop in the fluid that may increase further down from the pump. The pressure loss due to friction is commonly referred to as “friction loss” or “drag.” Such friction losses result in inefficiencies that increase equipment and operations costs and may place an upper limit on the achievable pumping rate. Due to that loss in pressure, additional mechanical equipment is often necessary to keep the fluid moving at the desired rate in the conduit. However, it is not always feasible to add the additional equipment needed to overcome these losses. An alternative to additional equipment involves introducing a substance to the fluid to reduce the friction itself. Such substances, known as drag-reducers, should reduce the friction loss of the fluid being transported, should be compatible with that fluid, and should not interfere with the intended use of the fluid. Ultra-high molecular weight polymers are known to function well as drag-reducing agents in aqueous fluids. However, traditional ultra-high molecular weight polymers have been found to be ineffective drag-reducers in fluids comprising carbon dioxide.